1. Field of the Invention
The present invention relates to a shadow mask for a flat cathode ray tube and, more particularly, to determining the proper installation position of a damper wire for mitigating vibrations of a shadow mask.
2. Discussion of Related Art
A typical flat cathode ray tube, as shown in FIG. 1, includes: a panel 1 whose internal surface is coated with R, G and B fluorescent coatings; an anti-explosion glass 2 fixed to the front surface of the panel 1 with a resin; a funnel 3 adhesively melted to the rear end of the panel 1 and held in a vacuum inside of the cathode ray tube; electron guns 5 embedded in the neck 4 of the funnel 3 with a seal; a shadow mask 7 having a plurality of pores 6 through which the electron beams emitted from the electron guns 5 pass and the electron beams are discriminated in colors; and a rail 8 for supporting the shadow mask and separating the panel 1 and the shadow mask 7 from each other at a fixed distance.
When power is applied to such a conventional flat cathode ray tube as constructed above, electron beams are emitted from the electron guns 5, discriminated in colors while they are passing through the shadow mask 7, striking the fluorescent coatings to create a picture.
The electron beams that pass through the pores of the shadow mask 7 are no more than about 20% with respect to the total electron beams emitted from the electron guns 5, and the rest of the electron beams strike the shadow mask 7 to generate heat energy which expands the shadow mask 7. This thermal expansion of the shadow mask 7 is referred to as a xe2x80x9cdoming effectxe2x80x9d.
The doming effect changes the positions at which the electron beams land on the fluorescent coatings, thereby deteriorating the purity of colors. In order to settle this problem, use is made of an invar mask less susceptible to thermal expansion for the shadow mask 7, or a bimetal spring for fixing the shadow mask 7 to the panel 1. However, such methods of suppressing the doming effect result in higher production cost and a deterioration of production efficiency.
Accordingly, a flat foil tension mask has been recently used to prevent a thermal expansion of the shadow mask 7 caused by the electron beams in a such manner that the shadow mask 7 has a stress to cause a compulsory elongation, and this mechanical deformation countervails against the thermal expansion of the shadow mask 7 resulting from striking electron beams.
Although such a shadow mask 7 is advantageous in prevention of the doming effect, the shadow mask 7 is no more than about 20 to 30 xcexcm in thickness and is thus susceptible to vibrations from insignificant shocks. And worse, there occurs a howling effect by which the vibrations of speakers shakes the shadow mask in operation of the cathode ray tube because the shadow mask 7 is soldered to the four sides of the rail 8 and the vibrational energy is easily transferred. The howling effect is a phenomenon that an external vibration such as caused by the sound pressure of speakers is iteratively transferred to the panel, spring, rail and shadow mask in sequence and thereby a resonance occurs to increase the amplitude of vibration of the shadow mask when the external vibration corresponds to the natural frequency of the shadow mask.
During the operation of the cathode ray tube in case of the howling effect, electron beams emitted from the electron guns 5 pass through the pores 6 of the shadow mask 7 but, when the shadow mask 7 vibrates, the electron beams strike the incorrect pixels. This is what is called a xe2x80x9cmislanding effectxe2x80x9d.
In order to avoid such vibrations of the shadow mask 7, a damper wire 9 with a tension is installed in a transverse direction of the shadow mask 7, as illustrated in FIG. 2. However, the damper wire 9 causes the pores 6 of the shadow mask 7 to be blocked and the corresponding fluorescent materials hardly emit lights, so that there remains a mark of the damper wire 9 on the screen, deteriorating the quality of picture.
According to the present invention, as shown in FIG. 3, a damper wire for supporting the shadow mask is disposed in a space between the pores 6 of the shadow mask 7 in order to prevent the pores 6 from being blocked by the damper wire 9, thereby maintaining a good quality picture and reducing the vibration of the shadow mask 7.
It is very important in this case to position the damper wire 9 accurately in a narrow space between the pores 6 of the shadow mask 7. Otherwise, the shadow of the damper wire 9 is thrown on the screen.
Therefore, a proper and acceptable reference position must be determined on the shadow mask so as to install the damper wire at the right position. Customarily, the reference position is determined with the naked eye and the damper wire is installed in a pore already formed on the shadow mask. However, it takes too much time and effort in positioning the damper wire accurately and causes an increase product defects.
Accordingly, an object of the present invention is to position a damper wire accurately in a space between the pores of a shadow mask without blocking the pores with the damper wire.
In accordance with an aspect of the present invention, a shadow mask for a flat cathode ray tube has a plurality of pores for discriminating electron beams emitted from electron guns by colors, and a vibration attenuation device disposed in a space between the pores. The shadow mask is provided with at least one reference portion for determining the position at which the vibration attenuating device is to be installed.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.